FIG. 1 is a view showing an example of the configuration of a conventional headphone drive circuit.
This headphone drive circuit comprises an amplifier 10R for amplifying power of a right-side signal SR, and an amplifier 10L for amplifying power of a left-side signal SL, and output signals of the amplifiers 10R, 10L are fed to a headphone 30 via respective capacitors 20R, 20L, for coupling.
The amplifiers 10R, 10L are of the same configuration, and, for example, the amplifier 10R has a terminal 11R for receiving the right-side signal SR, the terminal 11R being connected to a negative (−) input terminal of a differential amplifier 13R via a resistor 12R. An output of a differential amplifier 13R is connected to the negative (−) input terminal via a feedback resistor 14R. Further, a reference voltage Vref is delivered to a positive (+) input terminal of the differential amplifier 13R. In addition, the output of the differential amplifier 13R is connected to a terminal 31R of the headphone 30 via the capacitor 20R.
The headphone 30 comprises the terminal 31R for receiving the right-side signal, a terminal 31L for receiving the left-side signal, and a terminal 32 connected to a common potential GND. A voice coil 33R extends between the terminals 31R, 32, and a voice coil 33L extends between the terminals31L, 32. Variation of a magnetic field, occurring due to electric current flowing through the voice coils 33R, 33L, causes vibrating plates 34R, 34L to vibrate, thereby outputting sound.
With the headphone drive circuit, the signals SR, SL, delivered to the terminals 11R, 11L are amplified in power through the agency of the amplifiers 10R, 10L, respectively, thereby outputting amplified signals that are outputs of the amplifiers 10R, 10L, respectively, as biased by the reference voltage Vref (for example, a voltage equivalent to ½ of a power source voltage VDD).
The signals sent out from the amplifiers 10R, 10L are transmitted to the terminals 31R, 31L of the headphone 30 via the capacitors 20R, 20L, respectively, to be delivered to one end of the voice coils 33R, 33L, respectively. The other end of the voice coils 33R, 33L, respectively, is connected to an installation potential GND via the terminal 32. As a result, only signals at frequencies within the speech band, after elimination of DC portions thereof, are impressed to the respective voice coils 33R, 33L, and converted into sound to be sent out.
The conventional headphone drive circuit, however, has had the following problem. That is, because the outputs of the amplifiers 10R, 10L, respectively, are biased by the reference voltage Vref, the capacitors 20R, 20L, for coupling, are required in order to block DC from flowing to the voice coils 33R, 33L, respectively. These capacitors need to have a large capacitance to allow speech signals at low frequencies to pass therethrough.
Assuming that, for example, input impedance Zi of a voice coil 33 is 32 Ω, and a cutoff frequency of a bypass filter comprising the input impedance Zi and a capacitor 20 is 20 Hz, capacitance required of the capacitor 20 is 250 μF. Since a capacitor of such a large capacitance can not be used as an element of an integrated circuit, the capacitor needs to be installed externally, thus causing problems such as inability of implementing circuit miniaturization, and reduction in cost.